perf: build frontend once on runner instead of in Docker

MAJOR PERFORMANCE IMPROVEMENT: Reduced frontend Docker build time
from ~20 minutes to ~30 seconds by building the SPA once on the
runner instead of twice (amd64 + arm64) in Docker.

Changes:
1. Release workflow now builds frontend on CI runner (native, fast)
2. Frontend artifact uploaded and downloaded in release job
3. Dockerfile.frontend simplified to just copy pre-built files
4. Multi-arch Docker build is now just copying files into nginx

Before:
- Docker builds frontend 2x (amd64 + arm64 with QEMU emulation)
- Each: pnpm install + pnpm build = ~10 min per arch
- Total: ~20 minutes for frontend image

After:
- Build frontend 1x on runner = ~2 min (native)
- Docker just copies files = ~30 sec (both architectures)
- Total: ~2.5 minutes for frontend image

Impact:
- 8x faster frontend builds
- Total release time reduced from ~25 min to ~7 min
- Lower resource usage (no QEMU emulation)

Files changed:
- .github/workflows/release.yaml: Enable node build
- Dockerfile.frontend: Remove build stage, expect pre-built files
- .goreleaser.yaml: Copy frontend/dist instead of full source

.github/workflows/release.yaml

@@ -23,6 +23,12 @@ jobs:
     uses: lukaszraczylo/shared-actions/.github/workflows/go-release-cgo.yaml@main
     with:
       go-version: "1.25"
+      # Enable frontend build
+      node-enabled: true
+      node-version: "20"
+      node-build-script: "cd frontend && npm install -g pnpm && pnpm install --frozen-lockfile && pnpm run build"
+      node-output-path: "frontend/dist"
+      node-cache-dependency-path: "frontend/pnpm-lock.yaml"
     secrets: inherit
 
   benchmark: